By Dr. Liji Thomas, MD
Aug 17 2020
Study published on the preprint server medRxiv* in August 2020
Study: England
Study: England's Lockdown vs. Sweden's Herd Immunity: A Comparison of the Daily New COVID-19 Cases and Related Deaths Using Comparative Interrupted Time Series Analysis.
Herd Immunity Vs. Lockdown
When faced with the pandemic, Sweden aimed at achieving conferring herd immunity. It was hoped this would indirectly protect the rest of the population by ensuring that the risk of being exposed to the virus was reduced since infection chains cannot propagate among those who are immune to the virus. Proponents of the herd immunity approach say that it is a preferred strategy since lockdown cannot prevent viral spread but only delays it, at an unnecessary cost both economically and socially.
The lockdown approach is recommended, on the other hand, because it does decrease the effective reproduction number – the average number of people infected by each infected individual. When fewer people are infected in this way, the transmission chain is broken, and there are fewer cases to treat as well. Despite much media debate about which measures are better, lockdown or herd immunity, and plenty of research using predictive methods or tools to measure the change in the reproduction number, there was a lack of definitive data on the effectiveness of lockdown measures.
The current study is based on a quasi-experimental technique called interrupted time series (ITS) analysis. Here, the outcome of interest is collected at multiple points in time, both before and after a specific intervention. The trend of the outcome over time is thus observed, to find the effect of the intervention. In studies where randomization cannot or should not be done, this is an alternative practice.
The History of the UK Lockdown
By June 3, 2020, the World Health Organization confirmed about 279,000 and 38,500 cases of COVID-19 in the United Kingdom (UK) and Sweden, respectively, with the number of deaths crossing 39,000 and 4,400 in these countries. England had its first two cases on January 31, 2020, with the first COVID-linked death being reported on March 2. This was followed by escalation, with the lockdown put in place on March 23.
Both England and Sweden initially opted for herd immunity, but England switched to the lockdown strategy later, partly due to a model which showed that without any intervention, about 510,000 people in the UK would die of COVID-19. The two countries were chosen for the head-to-head comparison of daily case and death counts because the delay in declaring a lockdown in the UK allowed the pre-lockdown picture in both countries to be highly similar – a prerequisite for making inferences based on this data.
With the lockdown, people in England were not able to travel out of their homes freely to work, to stores, or for social activities. This was intended to keep the virus from spreading rapidly between people in close contact.
The History of the Herd Immunity Approach in Sweden
In Sweden, the first case was reported on the same day as in the UK, and the first death on March 11. However, Sweden did not adopt the lockdown strategy, instead choosing to make the public responsible for their health by stringent recommendations to stay at home for work as well as avoid traveling, as far as possible, to maintain social distancing, and if above 70 or showing the least signs of COVID-19, to stay at home all the time. However, preschools and elementary schools remained open.
The researchers used the effect of lockdown in England on daily cases and the rate of new cases and deaths in the first 100 days of lockdown, relative to Sweden. They found that the incidence of cases in England over 30 days was over 20,000 cases per 10 million people, vs. 15,000 in Sweden. Thus, England had a relative case incidence of 1.32.
In England, the incidence rate was approximately 3,400 deaths per 10 million people, vs. 2,400 in Sweden. Thus, the relative death incidence was 1.42. The case fatality in England and Sweden was 17% and 16% respectively.